Commercial allergenic extracts are complex mixtures of molecules of which only a few have allergenic activities. Current standardization of allergenic extracts with RAST or ELISA inhibition is based on serum from allergic patients. These assays are not possible to define what is being measured apart from the potency of the extract relative to a reference standard. Sandwich ELISA utilizes two monoclonal antibodies that recognize different epitopes on a specific allergen. This method is specific, sensitive, fast, and reproducible. It measures the quantity of a specific allergen in the mixture. The content of Fel d I, Der fI/fII and Der pI/pII in CBER reference extracts of cat, mite Df and mite Dp has been determined by sandwich ELISA. There is no difference in sensitivity, linearity, or accuracy when either purified allergens or CBER reference extracts are used as standard in the assay. We used Sandwich ELISA to assess the stability of specific allergens in allergenic extracts. Stability study was conducted at 4 degrees C, room temperature (R.T.)37 degrees C and 50 degrees C. Fel d I was stable at all temperatures over a period of one year. Der fI /f II maintained its stability at 4 degrees C, R.T. storage is stable for 9 month only, the allergen concentration decreased steadily when stored at 37 degrees C, and lost all of the allergenic activity after 3 month storage at 50 degrees C. Der p I was stable at 4 degrees C only. The concentration of Der p II was reduced at all temperatures. The stability study is still ongoing. With the help of Dr. M. C. Kuo at Immulogic and Dr. Karpas at CBER, monoclonal antibodies against Fel d I have been screened and selected . Evaluation of CBER monoclonal antibodies for use in Fel d I sandwich ELISA assay indicated that the sensitivity may be comparable with current method, but the specificity and accuracy of the assay need further experiments to clarify.